The present invention relates generally to data signal demodulators, and more particularly to phase-encoded data signal demodulators that may be advantageously utilized for demodulating phase-encoded data signals transmitted on a noisy communication channel, such as a radio channel found in some mobile and portable radio telephone communication systems.
One of the many different prior phase-shift keying techniques is the well known "Manchester" coding technique, in which a logical one signal is characterized by a negative going transition at the mid-bit position, and a logical zero signal is characterized by a positive going transition at the mid-bit position of the encoded data signal. In prior communication systems utilizing the Manchester coding technique, a phase-encoded data signal typically includes a data word preceded by a bit synchronization word, as illustrated in FIG. 2. The bit synchronization word may be a one-zero dotting pattern having only mid-bit transitions for synchronizing a receiving demodulator. Upon synchronization to the phase-encoded data signal, prior demodulators have utilized various techniques for demodulating the phase-encoded data signal. For example, a matched filter may be utilized. In a matched filter, the phase-encoded data signal is coupled through integrate and dump circuitry operating at the data signal bit frequency, which integrates the phase-encoded data signal to provide a waveform that may be sampled near the end of each bit interval to provide the decoded data signal. Because sampling must be precisely performed, a matched filter is not well suited for demodulating phase-encoded data signals transmitted on noisy communication channels, since spurious noise signals may interfere wtih precise synchronization to the phase-encoded data signal resulting in skewed sampling.
According to another prior demodulating technique, the phase-encoded data signal may be sampled at a frequency much higher than its bit frequency. The phase-encoded data signal may then be decoded by ascertaining the order in which a number of consecutive samples are detected above a positive threshold and a number of consecutive samples are detected below a negative threshold. In a modification of this demodulating technique, one sample may be taken at the mid-position of the first half and the mid-position of the second half of each data bit, and compared in order to determine the logical state of the phase-encoded data signal. However, the foregoing sampling demodulators are likewise not well suited for demodulating phase-encoded data signals transmitted on noisy communication channels, since spurious noise signals may result in erroneous samples.